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Beginner's Python Cheat-Sheet

lakpa profile image LP ・4 min read

Variables & Strings

Variables are used to store values. A string is a series of characters, surrounded by single or double quotes.

Hello world

print("Hello world!")

Hello world with a variable

msg = "Hello world!"

print(msg)

Concatenation (combining strings)

first_name = 'albert'

last_name = 'einstein'

full_name = first_name + ' ' + last_name print(full_name)

Lists

A list stores a series of items in a particular order. You access items using an index, or within a loop.

Make a list

bikes = ['trek', 'redline', 'giant']

Get the first item in a list

first_bike = bikes[0]

Get the last item in a list

last_bike = bikes[-1]

Looping through a list

for bike in bikes:

print(bike)

Adding items to a list

bikes = []
bikes.append('trek')
bikes.append('redline')

bikes.append('giant')

Making numerical lists

squares = []
for x in range(1, 11):

squares.append(x**2)

List comprehensions

squares = [x**2 for x in range(1, 11)]

Slicing a list

finishers = ['sam', 'bob', 'ada', 'bea'] first_two = finishers[:2]

Copying a list

copy_of_bikes = bikes[:]

Tuples

Tuples are similar to lists, but the items in a tuple can't be modified.

Making a tuple

dimensions = (1920, 1080)

If Statements

If statements are used to test for particular conditions and respond appropriately.

Conditional tests

equals x == 42
not equal x != 42
greater than x > 42
or equal to x >= 42
less than x < 42
or equal to x <= 42

Conditional test with lists

'trek' in bikes
'surly' not in bikes

Assigning Boolean values

game_active = True
can_edit = False

A simple if test

if age >= 18:
print("You can vote!")

If-elif-else statements

if age < 4:
ticket_price = 0
elif age < 18:
ticket_price = 10
else:

ticket_price = 15

Dictionaries

Dictionaries store connections between pieces of information. Each item in a dictionary is a key-value pair.

A simple dictionary

alien = {'color': 'green', 'points': 5}

Accessing a value

print("The alien's color is " + alien['color'])

Adding a new key-value pair

alien['x_position'] = 0

Looping through all key-value pairs

fav_numbers = {'eric': 17, 'ever': 4} for name, number in fav_numbers.items():

print(name + ' loves ' + str(number))

Looping through all keys

fav_numbers = {'eric': 17, 'ever': 4} for name in fav_numbers.keys():

print(name + ' loves a number')

Looping through all the values

fav_numbers = {'eric': 17, 'ever': 4}

for number in fav_numbers.values():

print(str(number) + ' is a favorite')

UserInput

Your programs can prompt the user for input. All input is stored as a string.

Prompting for a value

name = input("What's your name? ")
print("Hello, " + name + "!")

Prompting for numerical input

age = input("How old are you? ")
age = int(age)

pi = input("What's the value of pi? ")
pi = float(pi)

While-Loops

A while loop repeats a block of code as long as a certain condition is true.

A simple while loop

current_value = 1

while current_value <= 5:

print(current_value)

current_value += 1

Letting the user choose when to quit

msg = ''

while msg != 'quit':

msg = input("What's your message? ")

print(msg)

Functions

Functions are named blocks of code, designed to do one specific job. Information passed to a function is called an argument, and information received by a function is called a parameter.

A simple function

def greet_user():
"""Display a simple greeting."""
print("Hello!")

greet_user()

Passing an argument

def greet_user(username):
"""Display a personalized greeting."""

print("Hello, " + username + "!")

greet_user('jesse')

Default values for parameters

def make_pizza(topping='bacon'):

"""Make a single-topping pizza.""" print("Have a " + topping + " pizza!")

make_pizza()
make_pizza('pepperoni')

Returning a value

def add_numbers(x, y):
"""Add two numbers and return the sum."""
return x + y

sum = add_numbers(3, 5)
print(sum)

Class

A class defines the behavior of an object and the kind of information an object can store. The information in a class is stored in attributes, and functions that belong to a class are called methods. A child class inherits the attributes and methods from its parent class.

Creating a dog class

class Dog():

"""Represent a dog."""

def init(self, name):

"""Initialize dog object."""

self.name = name

def sit(self):

"""Simulate sitting.""" print(self.name + " is sitting.")

my_dog = Dog('Peso')

print(my_dog.name + " is a great dog!")

my_dog.sit()

Inheritance

class SARDog(Dog):

"""Represent a search dog."""

def init(self, name):

"""Initialize the sardog."""

super().init(name)

def search(self):

"""Simulate searching.""" print(self.name + " is searching.")

my_dog = SARDog('Willie')

print(my_dog.name + " is a search dog.")

my_dog.sit()

my_dog.search()

I
Reading a file and storing its lines

Working With Files

filename = 'LP.txt'

with open(filename) as file_object: lines = file_object.readlines()

for line in lines:

print(line)

Writing to a file

filename = 'journal.txt'

with open(filename, 'w') as file_object:

file_object.write("I love programming.")

Appending to a file

filename = 'journal.txt'

with open(filename, 'a') as file_object:

file_object.write("\nI love making games.")

Exception Handling

Exceptions help you respond appropriately to errors that are likely to occur. You place code that might cause an error in the try block. Code that should run in response to an error goes in the except block. Code that should run only if the try block was successful goes in the else block.

Catching an exception

prompt = "How many tickets do you need? "
num_tickets = input(prompt)

try:
num_tickets = int(num_tickets)
except ValueError:
print("Please try again.")

else:
print("Your tickets are printing.")

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@lakpa

I once single-highhandedly defended a small village in the Amazon Basin from a horde of ferocious army ants using only a hoe and a large glass of water

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